Apparatus for analysing suspended particles

ABSTRACT

Fibrous particles are distinguished from the other particles by illuminating them, detecting scattered light, and simultaneously applying an electrostatic field, the orientation of which is oscillated or rotated. Fibers tend to align with the moving field and thus produce a &#39;&#39;&#39;&#39;twinkle&#39;&#39;&#39;&#39; in the scattered light.

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[54] APPARATUS FQR ANALYSING [5 6] Reierences Cited SUSPENDED PARTICLES UNITED STATES PATENTS [72 Inventor: John Norman Chub! H d' 1 England ea mgto" 3,536,898 10/1970 Goldberg ..356/l03 3,257,903 6/1966 Marks ..350/160 X [73] Ass1gnee: United Kingdom Atomic Energy Authority, London, England Primary Examiner-Ronald L. Wibert Assistant Examiner-Conrad Clark [22] Wed 1971 Attorneyl..ars0n, Taylor & Hinds [21] Appl. No.: 114,983

[57] ABSTRAQT Foreign Application Priority Data Fibrous particles are distinguished from the other particles by illuminating them, detecting scattered light, Feb. 12, 1970 Great Britain ..6,926/ and simultaneously applying an electrostatic field, the orientation of which is oscillated or rotated. Fibers y 'fi tend to align with the moving field and thus produce a Q i u lht- 58 new 01 Search ..356/l03; 350/ twmkle mthescattered 6 (Ilairns, 1 Drawing Figure 3692412 on In. 356/103 APPARATUS FOR ANALYSING SUSPENDED PARTICLES BACKGROUND OF THE INVENTION The invention relates to apparatus for analyzing particles suspended in a fluid, more particularly airborne particles, in which it is desired to distinguish elongated particles, such as fibers, from spherical particles, or particles (hereinafter referred to as symmetrical particles) the appearance of which is substantially independent of the particle orientation.

SUMMARY OF THE INVENTION The invention provides apparatus for analysis of particles suspended in a fluid wherein the particles are detected by the radiation they scatter from a source of radiation, and wherein means are provided for subjecting the particles to an electrostatic field, whereby elongated particles, such as fibers, are distinguishable from symmetrical particles by their alignment with the electrostatic field.

In a preferred construction of apparatus according to the invention, particles within a defined region of the fluid are illuminated with light and a photodetector is positioned to detect light scattered by the particles.

Preferably distinction between elongated particles and symmetrical particles is effected by moving, preferably oscillating or rotating, the orientation of the electrostatic field in the plane of observation, whereby light scattered into the detector from elongated particles is modulated.

The light scattered in preferential directions according to the geometric form of the particle may be caused to enter the optical system of the photodetector at varying efficiency and produce a modulation of the electrical output of the photodetector related to the form of the particle.

The time taken for an elongated particle to align itself with an electrostatic field is dependent upon the particle length, the time taken being shorter for shorter particles. Consequently, for a given frequency of oscillation or rotation of the direction of the electrostatic field, only elongated particles of less than a certain length will follow the field. Accordingly it is a feature of the present invention to provide means for varying the frequency of oscillation or rotation of the direction of the electrostatic field for providing an indication of the length or lengths of elongated particles under observation.

In this connection, the presence of unipolar electrostatic charges upon the fibers disturbs the relationship between fiber length and time taken to align with a steady electrostatic field. This problem may be overcome by replacing the steady electrostatic field with a high frequency electrostatic field. The frequency has to be in excess of the migration rate of surface charge on the fibers.

For providing the aforesaid distinction between elongated particles and symmetrical particles, the orientation of the high frequency electrostatic field is oscillated or rotated at the, much lower, frequency appropriate for causing the particles to move to follow the orientation of the high frequency electrostatic field. If the frequency of this low frequency oscillation or rotation imposed upon the orientation of the high frequency field is varied, an indication may be derived of the length or lengths of elongated particles under observation.

In an alternative arrangement, the orientation of the high frequency electrostatic field is moved in sharp steps, whereupon elongated particles having certain dimensions will oscillate about the field alignment position and the particle length may be deduced from the period of oscillation revealed from measurement upon the photodetector output. Some particles will be sufficiently damped as to move in dead beat fashion into alignment. Information about the aspect ratio of these particles may be derived from the time taken to reach alignment.

BRIEF DESCRIPTION OF THE DRAWINGS A specific construction of apparatus embodying the invention will now be described by way of example and with reference to the accompanying drawing, which is a diagrammatic perspective view of the apparatus.

DESCRIPTION OF PREFERRED EMBODIMENT The apparatus of this example is intended for continuous analysis or monitoring of the content of suspended fibers in air, possibly .in the presence of higher concentrations of relatively symmetrical particles. The apparatus is particularly applicable to the monitoring of airborne asbestos fibers in factory and workplace environments.

Referring to the drawing, air to be sampled is drawn, in the direction indicated by arrow A, into an inner duct 11 which guides the air towards a test region 12. An outer duct 13 surrounding the inner duct 11 is connected via pipeline 14 to a source of clean air. Clean air from this source is drawn, as indicated by arrow B, into the outer duct 13 and passes through flow straighteners 15 to form a sheath of clean air at 16 surrounding the central air sample 17 in the test region 12. From there, all the air is exhausted to a pump via pipe 21, as indicated by arrow C.

The test region 12 is illuminated, to a closely defined depth, by a light source 18 and lens 19 with appropriate aperture stopping (not shown). An optical dump 22 is provided opposite the light source 18 so that disturbing reflections from enclosure 23 are avoided.

A photomultiplier 24 is mounted within the enclosure 23. An optical system including lens. 25 focuses onto the photomultiplier 24 light scattered from a closely defined area in the illuminated test region 12. This scattered light is observed against the dark background provided by the enclosure 23.

By arranging the illuminating beam to be well defined in depth in the direction of observation, the area concentration of particle images at the photomultiplier is directly related to the volume concentration of particles in the illumination region. Alternatively this condition may be secured by arranging to have the depth in the direction of observation of the sampled air stream well defined within a substantially uniform beam of illumination. The provision of a clean air sheath coupled with careful aerodynamic design of the ducts 11 and 13 provides that the concentration of particles in the observation region corresponds with that in the external region being sampled.

For enabling fibers to be distinguished from symmetrical particles, means are provided for imposing an oscillating or rotating electrostatic field, the direction of which is maintained, in this example, substantially parallel to the plane containing the axes of illumination and observation. In this example, the means comprises four electrodes 26, 27, 28, 29 positioned symmetrically about the test region 12, together with appropriate electrical supplies.

By applying suitable alternating potentials to the four electrodes, an electrostatic field which oscillates or rotates may be set up in the test region. For example, the diagonally opposed electrodes 26, 23 may be connected respectively to the ends of the secondary winding, with earthed center tap, of a transformer suitably driven with controlled frequency alternating current.

If electrodes 27, 29 are driven in a similar manner but with the secondary winding voltage so controlled as to be out of phase with respect to the drive to electrodes 26, 28, then the electrostatic field experienced by particles in the test region will appear to rotate.

Elongated particles which rotate with this field will be observed to twinkle," that is, the output from the photomultiplier will be modulated at the frequency of rotation of the electrostatic field.

Since, as indicated above, the maximum length of elongated particle that will follow an oscillating or rotating field is related to the frequency, a sweep of excitation frequency will provide for an approximate classification of the sizes of individual fibers within the illuminating beam. Thus the apparatus provides for generating a histogram of fiber length/concentration distribution in addition to the total fiber count within a given length range.

However, it has been found that the relationship between fiber length and rate of fiber alignment with an applied electrostatic field is upset by the presence on some fibers of unipolar electrostatic charge.

To avoid this problem, the steady electrostatic field is desirably replaced with a high frequency electrostatic field. The high frequency has to exceed the migration rate of surface charge on the fibers (typically 10' to 10 sec). In one arrangement a frequency of 700 kilohertz was employed, the electrostatic field strength being about 3.5 kVcm". A higher electrostatic field strength of around 10 kVcm' may be employed, with advantage, as indicated below.

The oscillation or rotation (at lower frequency) of the orientation of the high frequency field remains effective to cause the elongated particles to tend to follow the changing orientation of the high frequency field. Thus, a sweep of this lower frequency of oscillation or rotation of the orientation of the high frequency field will provide for approximate classification of the sizes of individual fibers within the illuminating beam, as stated above.

The following further observations on fiber behavior have been made:

a. While large diameter fibers oscillate about the applied field direction, the damping on the smaller diameter fibers is sufficiently strong to make their behavior dead beat,

b. In general terms, fibers of diameter greater than about 2 microns at 100 microns long, and 0.3 microns at 5 microns long exhibit oscillatory behavior for a permittivity around 7 and an electric field of 10 kVcm". Below these diameters the behavior is effectively dead beat." In practice the ability to distinguish oscillatory from dead beat behavior will depend upon the width of the light scattering pattern and the aperture of the collecting optics. The diameters at which oscillatory behavior is observed are larger at lower fields and at lower permittivities;

c. For oscillatory behavior the frequency of oscillation varies inversely with fiber length, is independent of fiber diameter and increases roughly as the three-halves power of the field;

d. For dead beat" fiber behavior similar time versus angle relationships can be obtained with various length diameter combinations, but no way of separating the length and diameter information on fibers exhibiting this behavior has yet been found. The fraction of fibers exhibiting this behavior may be small in practice and could be made smaller by operating at high electric fields;

e. The time scale for alignment at 10 kVcm' ranges from around 2.10 sec to 10 sec.

With these observations in mind, a preferred technique for assessing fiber dimensions is to square wave modulate the high frequency electrostatic field between the two main directions of alignment provided by the quadrupole electrode system 26, 27, 28, 29.

The modulation frequency should be sufficiently low for the longest and thinnest fibers likely to be monitored to have enough time for alignment within each modulation half cycle. The modulation frequency may range from Hertz to 50 ltilohertz and might typically be 1,000 Hertz for an electric field strength of 10 kl/cm' Desirably the electrodes are arranged so that in one of the directions of orientation of the high frequency electrostatic field, the aligned fibers are at the angle for maximum scatter of the illuminating light beam into the photomultiplier detector.

If the signal exhibits a series ofmaxima after the electric field is switched to the direction of maximum scattering intensity, the length of the oscillatory behavior fiber may be found using digital techniques to measure the oscillation frequency. If only a single maximum is observed, then the time for this to appear, and to disappear when the field direction is switched, will give some indication of the aspect ratio of the dead beat fiber observed. The information processing unit needed to interpret these photomultiplier signals becomes appreciably more complex if two or more fibers are present in the observation region at the same time. One way in which this problem can be minimized is by keeping the observation volume suitably small in relation to the fiber concentration being monitored.

To avoid double counting of individual fibers it is necessary that the modulation cycle time is not less than the transit time in the observation region. This can be checked and the volume of air sampled measured by observing the airflow velocity in the observation region. This may be done, for example, by observing on the main, or an auxiliary photomultiplier the transit of fiber or particle images across a coded grey filter band.

To avoid confusing the signals arising from the entry of bright particles into the observation field with dead beat fibers becoming aligned on the direction of maximum scatter, ramp function variations in illuminating intensity, or detection efficiency, may be provided for the regions of the observation zone where particles and fibers enter and leave. This would limit the maximum rate of change of signal in these regions.

To obtain sharp photomultiplier pulses from twinkle images it may be useful to use polarized illumination and to use fiber optics, rather than conventional lens optics, to collect the scattered light from a thin shell of the conical scattering surface.

it is considered feasible to distinguish fibers from other particles which are not symmetrical particles (as herein defined), for example from platelets, by the different form of light modulation produced. Thus, scattered light from airborne particles of platelet form is likely to twinkle in a similar manner to fibers, but the scattered light is restricted to the plane of rotation. As the scattering pattern for fibers includes directions above and below this plane the instrument can be made selective to fibers by excluding light scattered at angles close to the plane of rotation of the electric field.

The apparatus of this example is suitable for distinguishing elongated particles or fibers from symmetrical particles provided the fiber length is greater than the wavelength of the light used for illuminating the test region. It does not, however, matter if the fiber diameter is less than the light wavelength.

The invention is not restricted to the details of the foregoing example. For instance, the electrostatic field may be generated by electromagnetic waves generated in the test region by induction or by coupling from a transmission line or waveguide. The plane of oscillation or rotation of the electrostatic field need not necessarily be parallel with the plane containing the axes of illumination and observation, because light is scattered by particles in other directions.

I claim:

1. Apparatus for analysis of particles suspended in a fluid, which apparatus comprises a source of radiation, a radiation detector positioned to detect radiation scattered by the particles, means for generating an electrostatic field in the region of the particles, means for moving the orientation of the electrostatic field, whereby light scattered into the detector from elongated particles is modulated while light scattered into the detector from symmetrical particles is not so modulated, and means for indicating when detected scattered light is modulated.

2. Apparatus as claimed in claim 1, wherein the said source is a source of light, which illuminates particles within a defined region of the fluid, and a photodetector is positioned to detect light scattered by the particles.

3. Apparatus as claimed in claim 1, wherein the said means for moving the orientation of the electrostatic field comprises means for imparting an oscillation or rotation of the electrostatic field in the plane of observation.

4. Apparatus as claimed in claim 1, wherein the said means for generating the electrostatic field generates a high frequency field, the elongated particles tending to align with the orientation of the high frequency field, the frequency of which is in excess of the migration rate of unipolar electrostatic charges on the particles.

5. Apparatus as claimed in claim 4, wherein means is provided for oscillating or rotating at lower frequenc he orientation of the high frequency electrostatic fiel and frequency control means varies the frequency of the said oscillation or rotation for providing an indication of the length or lengths of elongated particles under observation.

6. Apparatus as claimed in claim 4, wherein means is provided for oscillating or rotating the orientation of the high frequency electrostatic field in sharp steps with a dwell at each step position, whereby information about the lengths or dimensions of elongated particles under observation may be derived from their periods of oscillation or time taken to reach alignment with the field during the dwell periods. 

1. Apparatus for analysis of particles suspended in a fluid, which apparatus comprises a source of radiation, a radiation detector positioned to detect radiation scattered by the particles, means for generating an electrostatic field in the region of the particles, means for moving the orientation of the electrostatic field, whereby light scattered into the detector from elongated particles is modulated while light scattered into the detector from symmetrical particles is not so modulated, and means for indicating when detected scattered light is modulated.
 2. Apparatus as claimed in claim 1, wherein the said source is a source of light, which illuminates particles within a defined region of the fluid, and a photodetector is positioned to detect light scattered by the particles.
 3. Apparatus as claimed in claim 1, wherein the said means for moving the orientation of the electrostatic field comprises means for imparting an oscillation or rotation of the electrostatic field in the plane of observation.
 4. Apparatus as claimed in claim 1, wherein the said means for generating the electrostatic field generates a high frequency field, the elongated particles tending to align with the orientation of the high frequency field, the frequency of which is in excess of the migration rate of unipolar electrostatic charges on the particles.
 5. Apparatus as claimed in claim 4, wherein means is provided for oscillating or rotating at lower frequency the orientation of the high frequency electrostatic field, and frequency control means varies the frequency of the said oscillation or rotation for providing an indication of the length or lengths of elongated particles under observation.
 6. Apparatus as claimed in claim 4, wherein means is provided for oscillating or rotating the orientation of the high frequency electrostatic field in sharp steps with a dwell at each step position, whereby information about the lengths or dimensions of elongated particles under observation may be derived from their periods of oscillation or time taken to reach alignment with the field during the dwell periods. 